Conventionally, a spark plug has been used to ignite an air-fuel mixture through spark discharge (which may be referred to merely as “discharge”) for operation of an engine, such as an internal combustion engine for an automobile. In recent years, high output and low fuel consumption have been required of internal combustion engines. To fulfill such requirements, use of a plasma jet ignition plug is known, since the plasma jet ignition plug provides quick propagation of combustion and exhibits such high ignition performance as to be capable of reliably igniting even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
When such a plasma jet ignition plug is used while being connected to a power supply, a spark discharge gap is formed between a center electrode and a ground electrode. The plasma jet ignition plug has a structure in which an insulator formed from ceramics or the like surrounds the spark discharge gap, thereby forming a small-volume discharge space called a cavity. A plasma jet ignition plug used with a superposition-type power supply (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-327672) is described by way of example. For ignition of an air-fuel mixture, first, high voltage is applied between the center electrode and the ground electrode, thereby generating spark discharge (also called “trigger discharge”). By virtue of associated occurrence of dielectric breakdown, current can be applied between the center electrode and the ground electrode with a relatively low voltage. Thus, through transition of a discharge state effected by further supply of energy, plasma is generated within the cavity. The generated plasma is jetted out through a communication hole (a so-called orifice), thereby igniting the air-fuel mixture. This process corresponds to a single cycle of jetting-out of plasma.
In generation of plasma, such a plasma jet ignition plug requires application, to the spark discharge gap, of current greater than that applied for generation of spark discharge in an ordinary spark plug. In order to increase current to be applied, electric resistance of a circuit through which the current flows must be lowered. Thus, there has not been an idea of providing a resistor in the interior of a plasma jet ignition plug (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 57-28869).